Store downloaded data in $XDG_DATA_HOME if that environment variable exists

[namurphy]

The XDG Base Directory Specification "defines a set of standard paths for storing application files, including data and configuration files." For example, defining $XDG_CACHE_HOME$ will let us change the location where cached files get put.

Currently, data files that get downloaded end up being stored by default in ~/.plasmapy. It would be helpful to provide users with a way to control where the files get downloaded to. I'm thinking we could make it so that if $XDG_CACHE_HOME$ exists, then we could make it so that the data gets downloaded to a directory more or less like $XDG_CACHE_HOME/plasmapy/downloads.

We should make sure to describe this in the documentation too.

[namurphy]

I just noticed that using this specification was brought up in this comment years ago!